The invention is applicable to high frequency oscillators which include, assembled on a sealed structure, a piezoelectric crystal that is connected to at least two exciter electrodes which produce an electric field in the active part of the piezo-electric crystal. Means are provided for applying to the electrodes on the one hand electric excitation power to vibrate the crystal according to a useful vibratory mode selected to determine a frequency reference, and on the other hand electric internal heating power to excite the crystal according to a second vibratory mode consisting of a non-harmonic overtone vibration which is distinct from the useful vibratory mode. The electric internal heating power is typically regulated with a temperature pick-up, which is located on the resonator or in its vicinity, in order to maintain the temperature of the resonator constant at the crystal reversal point.
Such an oscillator structure is described for instance in the French patent application No. 79 18553, now French Pat. No. 2462055, entitled "Self-Thermostated High Frequency Oscillator". Because the electric heating power is directly applied on the resonator, it is possible to limit the necessary energy expenditure for operating the oscillator. Then, the oscillator no longer has to be equipped with a conventional thermostat which is set at a temperature that is lower than the reversal temperature of the resonator crystal, for example, set at 50.degree. C. if the reversal point (PI) of the crystal is 85.degree. C., or at 35.degree. C. if the PI of the crystal is 70.degree. C.
However, in order for the vibration which is used as a frequency reference to be sufficiently accurate, it is necessary for the isochronism deficiency, also called amplitude-frequency effect or A.F. effect, to be as weak as possible and for the decoupling between both vibrations selected for the frequency reference and for internal heating to be as strong as possible.
Those conditions cannot always be achieved in practice. For instance, it is often necessary to build an oscillator around a resonator having a crystal which is of a single rotation section, such as an AT section. Indeed, this type of section displays advantages which are significant, such as manufacturing ease, low cost, low hysteresis deficiency and the fact that the reversal point of their temperature-frequency characteristic is high (greater than 80.degree. C.). And yet, an AT section crystal is not easily corrected for isochronism deficiency, and decoupling between the different vibration frequencies is easily not achieved.